1. Field of the Invention
The present invention relates to a sheet stacking apparatus and an image forming apparatus, and more particularly, to a structure for enhancing stacking performance of sheets and sheet bundles to be discharged onto a discharge stacking portion.
2. Description of the Related Art
Conventionally, as an example of image forming apparatus such as a copying machine, a laser beam printer, a facsimile machine, and a multifunction peripheral having functions of those apparatus, there has been used an image forming apparatus including a sheet stacking apparatus configured to stack sheets. Examples of such a sheet stacking apparatus include a sheet processing apparatus configured to perform a binding process and the like on sheets subjected to image formation. The sheet processing apparatus includes an intermediate stacking tray provided therein so that multiple sheets are stacked onto the intermediate stacking tray. In this way, a sheet bundle is formed, and subjected to processes such as the binding process. Then, the sheet bundle is discharged onto a discharge stacking portion.
In the sheet processing apparatus, in a case where sheets or sheet bundles are sequentially discharged onto the discharge stacking portion, sheets or sheet bundles which are precedingly discharged and stacked on the discharge stacking portion may be pushed and moved by sheets or sheet bundles which are subsequently discharged. In this case, stacking performance of the sheets or the sheet bundles to be discharged onto the discharge stacking portion is deteriorated. Conventionally, there has been proposed a sheet processing apparatus including a pressing member configured to press the sheets or the sheet bundles, in which the pressing member is lowered at a time of sheet discharge so that an edge on one side of the sheets or the sheet bundles precedingly stacked on the discharge stacking portion is pressed from the top (refer to Japanese Patent Application Laid-Open No. 2010-195494).
In the conventional sheet stacking apparatus and image forming apparatus including the sheet stacking apparatus, a sensor detects that a stacking height of an upper surface of an uppermost one of the sheets has reached a predetermined height. In response to a detection signal from the sensor, a height position of the discharge stacking portion is controlled and maintained at a predetermined height. In the conventional sheet stacking apparatus, in a case where sheet bundles subjected to the binding process are stacked, when the sheet bundles are stacked under a state in which bound portions thereof are superimposed on each other, as illustrated in FIG. 19A, multiple sheet bundles Sa are stacked under a state in which a bound edge portion St side thereof swells in a thickness direction.
When the sensor detects the swelling part of the sheet bundles Sa, as illustrated in FIG. 19B, a position of a flat edge portion of the sheet bundles Sa subjected to the binding process is on a line P2. However, the detection signal from the sensor indicates that the stacking height of the upper surface of the uppermost one of the sheets corresponds a line P1. Therefore, a discharge stacking portion 9 is lowered by a predetermined amount. In FIG. 19B, a pressing member 8 presses, from the top, an edge on one side of the sheet bundle Sa precedingly stacked on the discharge stacking portion 9. The pressing member 8 is pivotable about a fulcrum 8a within a predetermined range in a vertical direction.
When the discharge stacking portion 9 is lowered irrespective of a state in which an actual stacking height of the upper surface of the uppermost one of the sheets corresponds to the line P2, a pressing force of the pressing member 8 when pressing the sheets decreases. For example, when the pressing member 8 is provided with a longer arm and configured to be pivoted by a larger amount, the sheet bundles Sa subjected to the binding process can be firmly pressed by the pressing member 8 even when the discharge stacking portion 9 is lowered.
However, sheets that have not subjected to the binding process may be stacked onto the discharge stacking portion 9, and an actual stacking height of the upper surface of the uppermost one of the sheets in this case corresponds to the line P1. When the pressing member 8, which is provided with the longer arm and configured to be pivoted by the larger amount, presses the sheets having the actual stacking height corresponding to the line P1, the sheets are pressed with an unnecessarily greater pressing force. In this case, a force of moving the sheets by an amount corresponding to an arrow F may be applied along a locus of the pressing member 8, which may disturb the stacked sheets. In this way, when the sheets and the sheet bundles subjected to the binding process are pressed with the pressing member 8, stacking performance deteriorates.